1. Field of the Invention
The present invention relates to a solid-state imaging device, a method of driving a solid-state imaging device, and an imaging apparatus.
2. Description of the Related Art
In recent years, an amplification-type image sensor is known as a solid-state imaging device suitable for application to an imaging apparatus, such as a video camera or a digital still camera. The amplification-type image sensor has a structure in which, in each pixel cell (unit pixel), a signal obtained by photoelectric conversion is amplified by a MOS transistor (amplifying transistor) and is then output.
In such an image sensor, a technique of suppressing noise components generated due to various factors is needed to obtain high-definition images. With development of a technique of suppressing noise components generated due to thermal noise or variation of an element, it is possible to suppress the noise components generated due to the thermal noise or the variation of an element. However, it is still difficult to suppress 1/f noise of an amplifying transistor from occurring. In recent years, particularly the 1/f noise is becoming a dominant noise component of an output signal. Accordingly, in a future image sensor with high sensitivity, it is important to suppress the 1/f noise of an amplifying transistor.
In the related art, in order to reduce the 1/f noise of a MOSFET, there has been reported a method in which an operation of sweeping out electrons/holes trapped in a channel by causing a transistor to be in an OFF state or a deep accumulation state is repeatedly performed (for example, refer to IEEE Journal of Solid-State Circuits, vol. 35, no. 7, JULY 2000 “Reducing MOSFET 1/f Noise and Power Consumption by Switched Biasing”).
A principle of reducing the 1/f noise will now be described.
When a transistor is biased by a predetermined voltage Vbias, a current flowing into a main electrode of the transistor has a value corresponding to a sum of a predetermined current Ib and a noise current inoise. In general, a current noise of a transistor varies depending on a frequency, as shown in FIG. 23. For example, the thermal noise, which is a typical noise, is distributed over a wide band, and a 1/f noise is dominant in a low-frequency band of 1 MHz or less.
In the related art disclosed in IEEE Journal of Solid-State Circuits, vol. 35, no. 7, JULY 2000 “Reducing MOSFET 1/f Noise and Power Consumption by Switched Biasing”, it is possible to periodically sweep out electrons/holes trapped in the channel by performing a pulse operation of causing a transistor to be in an OFF state or a deep accumulation state. As a result, 1/f noise components lower than a pulse frequency can be reduced.
FIG. 24 illustrates the frequency spectrum of relative noise power of 1/f noise in a case of applying a pulse signal with a frequency of 10 kHz and a duty cycle of 50%. In FIG. 24, there are shown noise power spectrum in a case in which a constant bias voltage is supplied, noise power spectrum in a case in which a pulse having an amplitude from a bias voltage to a threshold voltage VT is supplied, and noise power spectrum in a case in which a pulse having an amplitude from a bias voltage to a voltage of 0 V is supplied. It can be seen that an effect of reducing the 1/f noise is achieved in a range of 10 kHz or less.
The principle of reducing the 1/f noise described above is applied to reduce the 1/f noise of a pixel transistor in a CMOS image sensor (for example, refer to JP-A-2003-32554).
FIG. 25 is a circuit diagram illustrating the basic configuration in a related art disclosed in JP-A-2003-32554. In the related art disclosed in JP-A-2003-32554, a CMOS image sensor 100 includes unit pixels 200, which are arranged in a matrix and in a two-dimensional manner and each of which has a photodiode 201, a transfer transistor 202, a reset transistor 203, an amplifying transistor 204, and a selection transistor 205. In addition, the CMOS image sensor 100 has a configuration in which switches 102-1 to 102-m are respectively added to vertical signal lines 101-1 to 101-m, such that a voltage supplied through a terminal 103 is applied to each of the vertical signal lines 101-1 to 101-m when each of the switches 102-1 to 102-m is in an ON state.
At this time, the applied voltage is a voltage close to a power supply voltage. In addition, by using the applied voltage, it is possible to cause the amplifying transistor 204 within the pixel 200 to be in an OFF state or a deep accumulation state. In this case, the voltage is applied to a main electrode (source electrode) of the amplifying transistor 204 through the selection transistor 205.